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perl -we 'my $k = int(shift); my @a; my $l = 0; while (<>) { if ($l++ 
+< $k) { push @a, $_ } else { if ((my $j = rand($l)) < $k) { $a[$j] = 
+$_; } } } print @a;' 3 filename

$ cat a
one
...
   1578 one six two 
   1578 four one three 
$